The present invention relates to a variable displacement compressor used for vehicle air conditioners, and more specifically, to a device and a method for controlling the displacement of a compressor.
In a general variable displacement compressor used for vehicle air conditioners, the inclination angle of a swash plate provided in a crank chamber changes in accordance with the pressure in the crank chamber. The crank chamber is connected to a suction chamber through a bleed passage and also to a discharge chamber through a supply passage. In the bleed passage is provided a displacement control valve. A controller containing a computer controls a control valve to adjust the amount of refrigerant gas that flows out into the suction chamber from the crank chamber through the bleed passage. As a result, the amount of the refrigerant gas which flows out of the crank chamber changes relative to the amount of refrigerant gas which is supplied to the crank chamber from the discharge chamber through the supply passage so that the pressure in the crank chamber is adjusted.
The control valve is provided with, for example, a valve body, a pressure sensing mechanism for operating the valve body in accordance with the pressure in the suction chamber (suction pressure), and an electromagnetic actuator, which urges the valve body with a force corresponding to the value of electric current supplied from the controller. The force of the electromagnetic actuator to urge the valve body reflects the target suction pressure. The controller adjusts the value of electric current supplied to the electromagnetic actuator to change the target suction pressure.
The controller increases the value of electric current supplied to the electromagnetic actuator to decrease the target suction pressure, and decreases the value of electric current supplied to the electromagnetic actuator to increase the target suction pressure. When electric current is not supplied to the electromagnetic actuator, the target suction pressure becomes a maximum value.
When a suction pressure exceeds the target suction pressure, the pressure sensing mechanism operates the valve body so as to increase the opening size of the bleed passage. Therefore, the flow rate of refrigerant gas from the crank chamber to the suction chamber is increased and the pressure in the crank chamber is then reduced. This increases the inclination angle of the swash plate so that displacement of the compressor increases. When the displacement of the compressor increases, the cooling performance of a refrigeration circuit incorporating the compressor increases and a suction pressure decreases so that it is converged to the target suction pressure.
When the suction pressure is lower than the target suction pressure, the pressure sensing mechanism operates the valve body to decrease the opening size of the bleed passage. Therefore, the flow rate of refrigerant gas from the crank chamber to the suction chamber decreases and the pressure in the crank chamber then increases. This decreases the inclination angle of the swash plate so that the displacement of the compressor decreases. When the displacement of the compressor decreases, the cooling performance of refrigeration circuit is reduced and a suction pressure increases so that it is converged to the target suction pressure.
Thus, the pressure sensing mechanism operates the valve body in accordance with the suction pressure in order to maintain the suction pressure at the target suction pressure.
The load on a vehicle engine increases under abrupt acceleration of the vehicle. Since the compressor is driven by the vehicle engine, if the engine load is great, the displacement of the compressor is temporarily minimized to reduce the engine load. Such displacement limiting control under abrupt acceleration of the vehicle will be described with reference to time charts of FIGS. 6(a) to 6(c).
As shown in FIG. 6(a), when a vehicle is abruptly to accelerated in a state where electric current of the predetermined value is supplied to an electromagnetic actuator of a displacement control valve, a controller sets the supplied current value for the electromagnetic actuator at zero to start the displacement limiting control. As a result, as shown in FIG. 6(b), the target suction pressure Pst is set at a maximum value Pmax. Then, the pressure sensing mechanism of the displacement control valve closes the bleed passage with the valve body to bring an actual suction pressure Psa near to the maximum value Pmax. Thus, the pressure in the crank chamber increases and the inclination angle of the swash plate becomes minimum, whereby the displacement of the compressor becomes minimum as shown in FIG. 6(c). In other words, the torque of the compressor becomes minimum so that the engine load is reduced.
When the target suction pressure Pst changes, some time is required for this change to be reflected in the change in the actual suction pressure Psa. Thus, when the target suction pressure Pst is rapidly changed to the maximum value Pmax as shown in FIG. 6(b), the actual suction pressure Psa gradually increases toward the maximum value Pmax.
As shown in FIG. 6(a), a displacement limiting control due to abrupt acceleration of a vehicle is completed after the lapse of the predetermined time S from its start. After that, the displacement limiting control is shifted to a usual displacement control in accordance with a cooling performance required for the refrigeration circuit. Specifically, the controller resumes the supply of current to the electromagnetic actuator after the lapse of the predetermined time S after setting the supplied current value for the electromagnetic actuator at zero. At this time, the controller obtains the target current value A3 according to the cooling performance required for the refrigeration circuit, and gradually increases the supplied current value for the electromagnetic actuator from zero to the target current value A3 for the predetermined time T (refer to the straight line H in FIG. 6(a)). According to this increase, the target suction pressure Pst gradually decreases from the it maximum value Pmax to the value P3 corresponding to the target current value A3 for the predetermined time T as shown in FIG. 6(b).
If the target suction pressure Pst rapidly decreases from the maximum value Pmax to the value P3, the actual suction pressure Psa, which is gradually increasing toward the maximum value Pmax, significantly exceeds the value P3 temporarily. Then, the pressure sensing mechanism of the displacement control valve causes the valve body to abruptly open the bleed passage to decrease the actual suction pressure Psa to the value P3. This leads to an abrupt decrease in the pressure in the crank chamber and rapidly increases the displacement of the compressor. As a result, the torque of the compressor rapidly increases and the engine load rapidly increases, whereby the vehicle drivability is deteriorated. To avoid such problems, the target suction pressure Pst gradually decreases from the maximum value Pmax to the value P3 for the predetermined time T.
As shown in FIG. 6(b), the actual suction pressure Psa is always lower than the target suction pressure Pst set at the maximum value Pmax through the predetermined time S when the displacement limiting control is being executed. Further, since the target suction pressure Pst gradually decreases at the completion of the displacement limiting control, the actual suction pressure Psa is still lower than the target suction pressure Pst between the completion of the displacement limiting control and the end of time Ta. When the time Ta elapses after the completion of the displacement limiting control, the actual suction pressure Psa substantially becomes equal to the target suction pressure Pst. After that, the actual suction pressure Psa is gradually reduced to the value P3 as the target suction pressure Pst is gradually reduced to the value P3.
When the actual suction pressure Psa is lower than the target suction pressure Pst, the pressure sensing mechanism of the displacement control valve causes the valve body to control the opening size of the bleed passage to increase the actual suction pressure Psa so as to bring it near the target suction pressure Pst. In other words, even if the displacement limiting control is completed, the pressure sensing mechanism does not execute an operation for decreasing the actual suction pressure Psa, that is an operation for increasing displacement of a compressor from the minimum state until the time Ta elapses after the completion.
In addition, the displacement control valve completely closes the bleed passage during execution of the displacement limiting control and the pressure in the crank chamber is excessively increased due to the high pressure gas supplied through the supply passage. Therefore, even if the control valve increases the opening size of the bleed passage to increase the displacement of the compressor after the lapse of the time Ta after the completion of the displacement limiting control, it takes much time to lower the pressure in the crank chamber to pressure by which the displacement of the compressor can shift from the minimum state to an increased state. Thus, as shown in FIG. 6(c), the displacement of the compressor shifts from the minimum state to the increased state with a delay of a considerably long time Tb after the completion of the displacement limiting control. That is, the displacement of the compressor is maintained in the minimum state for a time longer than the execution time S of the displacement limiting control. This means that a cooling performance of the refrigeration circuit unnecessarily decreases for a long time. As a result, the passenger compartment temperature further becomes higher than before execution of the displacement limiting control, which gives discomfort to passengers in the vehicle.
Accordingly, it is an objective of the present invention to provide a displacement control device and a displacement control method for a variable displacement compressor which smoothly and rapidly shift a displacement limiting control to a usual displacement control.
To attain the above-mentioned object, the present invention provides a displacement control device for a compressor that changes the displacement in accordance with the pressure in a control pressure chamber. The control device includes a control valve, a detector and a controller. The control valve controls the pressure in the control pressure chamber. The control valve has a valve body and an electromagnetic actuator for actuating the valve body. The actuator urges the valve body by a force the magnitude of which corresponds to the value of current supplied to the actuator. The detector detects external conditions that are necessary for controlling the compressor displacement. The controller controls the value of current supplied to the actuator. The controller selects a control mode to be executed from a usual displacement control and a displacement limiting control based on the detected external conditions. When the usual displacement control is selected, the controller sets the current value to a target value, which corresponds to the detected external conditions. When the displacement limiting control is selected, the controller temporarily sets the current value to a specific value to minimize the compressor displacement. When the control mode is switched from the displacement limiting control to the usual displacement control, the controller changes the current value from the specific value to the target value taking a predetermined restoration period. For at least part of the restoration period, the controller sets the current value to a value that is closer to the target value than a corresponding value on a direct proportional line, which represents a constant rate of change from the specific value to the target value.
The present invention also provides a method for controlling the displacement of a compressor that changes the displacement in accordance with the pressure in a control pressure chamber. The method includes: controlling the pressure in the control pressure chamber by a control valve, wherein the control valve has a valve body and an electromagnetic actuator for actuating the valve body, wherein the actuator urges the valve body by a force the magnitude of which corresponds to the value of current supplied to the actuator; detecting external conditions that are necessary for controlling the compressor displacement; selecting a control mode to be executed from a usual displacement control and a displacement limiting control based on the detected external conditions; setting the current value to a target value, which corresponds to the detected external conditions, when the usual displacement control is selected; temporarily setting the current value to a specific value to minimize the compressor displacement when the displacement limiting control is selected; and changing the current value from the specific value to the target value taking a predetermined restoration period when the control mode is switched from the displacement limiting control to the usual displacement control. For at least part of the restoration period, the current value is set to a value that is closer to the target value than a corresponding value on a direct proportional line, which represents a constant rate of change from the specific value to the target value.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.